Contact lenses are widely used to correct vision defects in mammalian wearers of the lenses. Vision defects include, for example, myopia, hyperopia, astigmatism, and presbyopia. Contact lenses can come in hard and soft varieties. Typically the hard variety is made from glass or a rigid polymer such as, for example, poly (methyl methacrylate ) (PMMA), poly (hydroxyethyl methacrylate) (HEMA) and polycarbonates. The soft lenses are usually made from a polymeric material.
Contact lenses have various corrective types, such as, single vision, bifocal, trifocal, and progressive. When contact lenses are placed in the eye, a tear layer separates the lens from the surface of the eye. This layer is commonly known as the tear film. The tear film contains water, proteins, lipids, sodium, calcium, bicarbonate, and enzymes. A contact lens can disrupt the tear film by repelling water due to its hydrophobic nature, which can lead to discomfort for the wearer The disruption in the tear film also can lead to the deposit of albumin on the lens, thus, reducing the effectiveness of the film. Prior-art attempts to mitigate the effects caused by hydrophobic lens material led to coating the hydrophobic material with a hydrophilic polymeric layer. Contact lenses that aggravate the dry eye condition are not recommended for extended use.
Because most lens materials are formed using monomers and cross-linking agents, which have electrostatic charges associated therewith, proteins, which also have charge distributions, are attracted to the lens. This can cause the lens to turn yellow in color and can reduce the corrective effects of the lens. Such a lens is not sufficiently biocompatible for extended use.
Because the anterior segment (the surface of the front of the eye, including the cornea) of the human eye does not receive adequate blood flow to supply it with oxygen and to remove carbon dioxide, the eye must have sufficient exposure to air to remain in a healthful condition. Thus, a contact lens must have sufficient permeability to the passage of oxygen to maintain good eye health. Permeability, DK, typically is measured in units of Barrers, where D is the diffusion coefficient, and K is the solubility coefficient. A contact lens should have a permeability, or a DK, of at least 100 Barrers to be suitable for extended use.
The contact lens material also must not pose a health risk and not cause an allergic reaction. The lens also must be sufficiently lightweight to be comfortable to wear and to avoid ocular strain. It is also necessary to be of the appropriate size to function properly and to be comfortable. The lens must also have the proper specific gravity so that the lens does not move around on the eye by floating on the tear film. The lens must also be of sufficient flexibility to conform to the eye surface and be resilient enough for a user of the lens to handle the lens without tearing or scratching. The lens also must have the proper thickness so as not to cause discomfort during blinking or to reduce the blink rate.
Contact lenses can be manufactured using several methods, including lathe cutting, spin casting, and cast molding. In a lathe-cutting process, a polymeric material is polymerized into rod-shaped stock material, that is cut into buttons and mounted in a lathe where it is cut into a lens. In the spin-casting method, reactive polymeric materials are placed in a mold which is rotated. The reactive liquids polymerize to form a solid contact lens. By varying the speed of rotation, the optics of the lens can be varied. In the cast-molding process, reactive monomeric material is placed into a mold. The mold is then cast, and the monomer polymerizes forming a solid lens. The optics of the lens are adjusted by varying the shape of the mold.
All prior art contact lens materials have an amorphous, three-dimensional cross-linked polymer matrix. Hard contact lens materials typically are polymers below their glass transition temperature and include very little or no water. Soft contact lenses are typically made from polymers above their glass transition temperature and have relatively high water content.